It has become standard surgical procedure to saw off the neck and head of a femur and replace them with a metal prosthesis that then acts as the upper swivel end of the femur in the acetabulum. This procedure can correct an arthritic problem, or simply replace badly broken parts of the femur.
The typical prosthesis for this surgical procedure comprises three parts: a ball head shaped to be received in the acetabulum, a stem extending from the head and forming a shoulder that bears against the sawed-off end of the femur, and a shank or shaft which extends down into the medullary cavity of the femur. The shaft or shank is typically slightly bent in a plane and the stem is bent away from the overall longitudinal axis of this shaft in order that the prosthesis accurately replace and conform to the shape of the sawed-off upper processes of the femur.
The shaft of the above-described prosthesis is set in the bored-out medullary cavity of the femur with the aid of bone cement. Thus standard procedure has always been to use a shaft which is spaced all around from the inside wall of the femur, filling the lateral interstices between the shaft of the prosthesis and the inside wall of the femur in the medullary cavity thereof with bone cement.
Such mounting has proven troublesome in many situations. In particular, due to the enormous stresses which this particular prosthesis is subjected to, so-called micromovements of the prosthesis relative to the femur often soon translate themselves into dangerous looseness. Thereupon it is necessary to operate again on the patient and reset the loose prosthesis.